The aim of this study is to develop equations governing left ventricular pressure and volume during a cardiac cycle. It is assumed that the stress in the myocardium is composed of an isotropic tissue pressure term and fiber tension. Fiber tension T depends on the stretching and rate of stretching of cardiac fibers as well as an internal variable c that describes the degree of electrochemical activation of the muscle fiber. Using mechancial equations of equilibrium and the constitutive equations mentioned above, a time dependent pressure-volume relation (P-V) is obtained for the left ventricle. Hydraulic characteristics of the large arteries are modelled by a three parameter windkessel model. It is assumed that the aortic pressure is equal to the ventricular pressure P when the aortic valve is open. Mechanical events of the cardiac cycle are considered as a function of heart rate by changing one of the following parameters: end diastolic volume EDV, contractility co time constant of contraction w, and resistance R and compliance C of the large arteries. For given EDV, an increase in HR leads to increases of both systolic pressure Ps and diastolic pressure Pd with a decrease of pulse pressure, and a decrease of ejection fraction, and a biphasic change in cardiac output CO, which increases at first to reach a maximum and then decreases. When w is doubled, the rate of pressure rise and maximum flow rate are approximately doubled, but there is very little change in stroke wolume SV, Ps and Pd for moderate HR. At higher HR levels, cardiac output CO increases with w because of the lengthening of diastolic phase; aortic valve opens at an earlier time. An increase in co increases CO as well as Ps and Pd, regardless of HR level. SV and CO vary inversely with R and directly with the slope of the isovolumetric P-V curve.